1. Field of the Invention
The invention relates to a method for static and dynamic support of a mains voltage at a grid system node by means of a static power factor correction device which has a transformer and a self-commutated converter with at least one capacitive energy store, and to an apparatus for carrying out the method.
Electrical power supply grid systems are used primarily for transmitting real power. The power produced and the power consumed must always be matched, otherwise frequency changes occur. In the same way as the real power equilibrium, the reactive power (volt-ampere power) equilibrium must also always be matched such that the voltage conditions in the grid system are acceptable. The reactive powers are primarily responsible for the voltage level. The real power and reactive power equilibrium in the grid system must be matched at all times such that the voltage and the frequency are within the predetermined limits.
As a result of increasing current consumption and limited grid system extension capabilities, the power transmission grid systems are being used more and more intensely. Reactive power flows in the grid system are the main cause of voltage drops and additional grid system losses. A matched reactive power equilibrium in the grid system, and thus the effects on the mains voltage and grid system losses, can be reduced by the deliberate use of reactive devices, such as capacitors and coils. The differing dynamic requirements can be covered by switchable or controllable reactive elements. However, in practice, continuous and dynamic changes are possible only by using converter circuits. Static power factor correction devices using thyristor technology represent the most economic solution for dynamic power factor correction at the moment.
The terms "Static Var Generator (SVG)", "Advanced Static Var Compensator (ASVC)" or "Static Condenser (STATCON)" are normally used for power factor correction devices which have a self-commutated converter with a capacitive energy store. Reactive powers at the fundamental frequency can be added or subtracted by means of the self-commutated inverter, which converts the DC voltage of a capacitive energy store (capacitor) into an AC voltage and is connected to a grid system node (PCC=Point of Common Coupling) via a transformer (reactance).
2. Description of the Related Art
A power factor correction device is disclosed in the article "A Comparison of Different Circuit Configurations for an Advanced Static Var Compensator (ASVC)" printed in "PESC` 92 Record; 23rd Annual IEEE Power Electronics Specialists Conference Toledo, Spain", 1992, pages 521-529. That article proposes a number of ASVC circuits and compares them with one another. The basic circuit of an ASVC comprises a three-phase inverter with a capacitive energy store. The inverter is connected to a grid system node (PCC) through a transformer.
The inverter of that system is either a two-point inverter or a three-point inverter. These inverters are conrolled by means of fundamental frequency modulation (full block control), the reactive powers which can be achieved at the fundamental frequency being greater in a three-point inverter than in a two-point inverter. In addition, the fifth and seventh harmonics are minimal if the angle .beta. (angle range for zero potential) is equal to .pi./12. In addition, that article investigates ASVC circuits which comprise two two-point or three-point inverters, different transformers being used. Those circuits are intended to reduce the pulse number and the distortion of the phase current.
A Static Var Generator (SVG) for 80 MVA is proposed in the article "Development of a Large Static Var Generator Using Self-Commutated Inverters for Improving Power System Stability", printed in "IEEE Transactions on Power Systems", Vol. 8, No. 1, February 1993, pages 371-377. The 80 MVA SVG comprises eight inverters, whose bridge paths each comprise six series-connected gate turn-off thryistors (GTO), which each produce the same output voltage, but respectively shifted through 7.5.degree. electrical with respect to one another. A special transformer with eight primary windings and eight secondary windings is required for that phase shift. The special transformer is connected to a high-voltage grid system by means of a main transformer. The transformer design requires a part of the reactive powers provided. A pulse number of 48 is thus obtained, and hence a reduction in the grid system feedback effects, while the fundamental power is at the same time increased. That power factor correction device (SVG) also achieves an improvement in the fundamental power only by using a plurality of inverters and a special transformer, but the fundamental yield per switching device of the inverter is not increased.
A further option for improving the fundamental yield (power per switching element) is to use a multipoint inverter. A five-point inverter for an SVC is proposed in detail in the article "A High Voltage Large Capacity Dynamic Var Compensator Using Multilevel Voltage Source Inverter", printed in "PESC` 92 Record, 23rd Annual IEEE Power Electronics Specialists Conference Toldedo, Spain", 1992, pages 538-545. The use of a mulitpoint inverter improves the fundamental yield for the AC voltage produced by the inverter.
The article "System Variable Evaluation With Digital Signal Processors for SVC Applications" by G. Welsh et al., printed in "Fifth International Conference on AC and DC Power Transmission", 1991, pages 255-260 discloses a voltage regulation system for a static power factor correction device (SVC) comprising a permanently connected capacitance (FC), a thyristor-controlled coil (TCR) and a thyristor-switched capacitance (TSC), which is constructed in digital form. That voltage regulation system uses the three line-ground voltages to determine a system voltage (mean value of the three line-line voltages, which are compared with a system reference voltage). The control error obtained is used to determine a control signal for the power factor correction device, by means of a voltage regulator at whose output susceptance value is produced. In that digital regulation system, the system voltage is determined using a signal processor, which is part of a multiprocessor control system (SIMADYN D).
The above-mentioned publication "PESC` 92 Record, 23rd Annual IEEE Power Electronics Specialists Conference Toledo, Spain", 1992, pages 538-545, proposes not only a five-point inverter for an SVC in detail, but also the associated control system. That control system has a device on the input side which is used to determine the mean value of a reactive power control error Q.sub.s as a function of a reactive power value Q.sub.L required by a load and a reactive power value Q.sub.I supplied by the self-commutated inverter.
A downstream PI regulator uses this value Q.sub.s to produce a manipulated variable .phi..sub.m (phase angle for the inverter). The drive element, which has optimized pulse patterns, uses the manipulated variable .phi..sub.m and a modulation value M.sub.i to generate control signals for the self-commutated inverter. That control system ensures that the reactive power control error Q.sub.S is reduced to zero. If the control error Q.sub.S =0, then the manipulated variable .phi..sub.m =0 and the output voltage of the inverter is in phase with the voltage at the coupling point PCC. The purpose of that control system is to supply the reactive powers required by the load from the self-commutated inverter. The control system therefore cannot statically or dynamically support the mains voltage at the coupling point (point at which the power factor correction device is connected to the grid system).